.SIGMA..DELTA. analog/digital converters are commonly used in modern signal processing systems for converting analog input signals into digital signals, such as, for example, in audio applications and video systems. A typical drawback of .SIGMA..DELTA. converters is the phenomenon of intrinsic generation of tones that may be reflected back in the band of interest, for example the audio band, in presence of low dynamic input signals. Particularly undesirable are the tones near half a value of the sampling frequency because they have the greatest energetic content and are also more likely to be reflected back in the audio band as a consequence of nonlinearities and intermodulation effects.
Dithering techniques are commonly used to curb these undesirable phenomena. A dither signal, injected in the modulator that converts the analog signal into digital, has the function of making the error of the comparator of the analog/digital converter, often referred to as the modulator, as independent as possible from the input signal. Thus, the generation of tones and the resulting speed of their energy on the whole band may be averted.
Dithering techniques normally used in .SIGMA..DELTA. analog/digital converters are based on the injection of deterministic signals, whose energy is spectrally distributed outside the band of interest. The negative effect of dithering techniques manifests itself when the input signals are near the maximum design dynamic. The energy of the dither signal summed to that of the input signal to be converted may bring the stages of the modulator to operate under saturation conditions. In order to avert this problem, the dither signal may be attenuated or even deactivated in the presence of high dynamic input signals, implementing what is commonly referred to as an adaptive dither.
In the case of audio band converters, the most common signals being used are relatively continuous or sinusoidal signals of a frequency sufficiently above the band of the analog input signals being subject to analog/digital conversion. Continuous dither signals have a scarce effect, while the use of sinusoidal signals require a considerably more complex circuitry. Dither signals with a spectrum similar to white noise, notably very efficient for eliminating the tones, may be generated only digitally and for this reason are generally employed in digital/analog .SIGMA..DELTA. converters. A dithering signal, with a spectrum similar to that of white noise, has never been applied to analog/digital .SIGMA..DELTA. converters.